Slicing method

The present invention relates to a slicing method for slicing, e.g., a silicon ingot or an ingot of a compound semiconductor into many wafers by using a wire saw.

In recent years, an increase in size of a wafer is demanded, and a wire saw is mainly used to slice an ingot with this increase in size.

The wire saw is a apparatus that allows a wire (a high-tensile steel wire) to travel at a high speed and presses an ingot (a work) against the wire to be sliced while applying a slurry to the wire, thereby slicing the ingot into many wafers at the same time (see Japanese Unexamined Patent Publication (Kokai) No. 262826-1997).

Here, FIG. 6 shows an outline of an example of a general wire saw.

As shown in FIG. 6, a wire saw 101 mainly includes a wire 102 that slices an ingot, grooved rollers 103 (wire guides) around which the wire 102 is wound, a mechanism 104 that gives the wire 102 a tensile force, a mechanism 105 that feeds the ingot to be sliced, and a mechanism 106 that supplies a slurry at the time of slicing.

The wire 102 is unreeled from one wire reel 107 and reaches the grooved rollers 103 through the tensile-force-giving mechanism 104 formed of a powder clutch (a constant torque motor 109), a dancer roller (a dead weight) (not shown) and so on through a traverser 108. The wire 102 is wound around this grooved rollers 103 for approximately 300 to 400 turns, and then taken up by a wire reel 107′ through the other tensile-force-giving mechanism 104′.

Further, the grooved roller 103 is a roller that has a polyurethane resin press-fitted around a steel cylinder and has grooves formed at a fixed pitch on a surface thereof, and the wire 102 wound therearound can be driven in a reciprocating direction in a predetermined cycle by a driving motor 110.

It is to be noted that such an ingot-feeding mechanism 105 as shown in FIG. 7 feeds the ingot to the wire 102 wound around the grooved rollers 103 at the time of slicing the ingot. This ingot-feeding mechanism 105 includes an ingot-feeding table 111 that is used to feed the ingot, an LM guide 112, an ingot clump 113 for grasping the ingot, a slice pad plate 114, and others, and driving the ingot-feeding table 111 along the LM guide 112 under control of a computer enables feeding the ingot fixed at the end at a previously programmed feed speed.

Moreover, nozzles 115 are provided near the grooved rollers 103 and the wound wire 102, and a slurry can be supplied to the grooved rollers 103 and the wire 102 from a slurry tank 116 at the time of slicing. Additionally, a slurry chiller 117 is connected with the slurry tank 116 so that a temperature of the slurry to be supplied can be adjusted.

Such a wire saw 101 is used to apply an appropriate tensile force to the wire 102 from the wire-tensile-force-giving mechanism 104, and the ingot is sliced while causing the wire 102 to travel in the reciprocating direction by the driving motor 110.

On the other hand, in a wafer, a size of a surface waviness component that is called “nano-topography” is a problem in recent years. This nano-topography is obtained by taking a wavelength component having a wavelength λ=0.2 mm to 20 mm that is shorter than “Sori” or “Warp” and longer than “surface roughness” out of a surface shape of a wafer. And, this nano-topography is very shallow waviness having a PV value of 0.1 μm to 0.2 μm or below. It is said that this nano-topography affects a yield of an STI (Shallow Trench Isolation) process in device manufacture.

Although the nano-topography is produced in a wafer processing step (slicing to polishing), it was revealed that a nano-topography caused due to wire saw slicing (i.e., slice waviness) can be classified into three types, i.e., “one that is extemporaneously produced”, “one that is produced in a position where slicing is started or ended”, and “one having a periodicity” as shown in FIG. 8.

Of these types, one that is produced in “slicing start/end portion of a wafer” has a high rate that it is rejected in a numeric judgment regarding a nano-topography. In particular, a nano-topography in the “slicing end portion” has a higher rate than a nano-topography in the “slicing start portion”. And the “slicing end portion” highly frequently becomes a position making a numeric value regarding a nano-topography the worst in a wafer radial direction or the nano-topography in the “slicing end portion” is rejected in the numeric judgment, and hence improvement is strongly demanded.

Thus, the present inventors examined nano-topographies in sliced wafers sliced by using such a conventional wire saw as shown in FIG. 6.

FIG. 9 shows Warp cross-sectional shapes measured by an electric capacitance type measuring instrument and “pseudo nano-topographies” of the sliced wafers. The pseudo nano-topography means obtaining a numeric value having a correlation with a nano-topography of a polished wafer in a pseudo manner by applying a band-pass filter having simulated processing characteristics of lapping, grinding, and polishing with respect to Warp cross-sectional wave shape of the sliced wafer.

In general, the nano-topography is measured after polishing but, when a pseudo nano-topography is obtained from the sliced wafer and the obtained pseudo nano-topography is used, a cost and a time do not have to be increased, and a nano-topography caused due to an influence of slicing alone can be readily examined without being affected by a factor in a process such as polishing after slicing.

It was understood from such an examination that a nano-topography near a slicing end portion that is demanded to be improved the most in the conventional technology is caused due to a precipitous change in a Warp shape of a wafer in this position.

FIG. 9(A) shows a wafer holding small change in shape in a position near a slicing end portion as depicted in a shape map, and a size of a change of a pseudo nano-topography is suppressed to the range of ±0.1 μm and relatively small in the position near the slicing end portion as can be understood from the pseudo nano-topography. On the other hand, as shown in FIG. 9(B) or FIG. 9(C), it can be understood that, when a shape in the position near the slicing end portion is precipitously changed, a size of a pseudo nano-topography falls within the range of −0.3 to 0.4 in this position and is larger than that depicted in FIG. 9(A).

It is to be noted that, if a change in the entire shape is gentle even though this change is slightly large, the nano-topography is hardly produced. A precipitous change in shape greatly affects the nano-topography.

Thus, a factor of generation of such a precipitous change in a sliced wafer in the position near the slicing end portion as shown in FIG. 9 was examined.